1. Field of the Invention
The present invention generally relates to a technique for reading an image formed on a medium using solid-state image pickup devices, such as line sensors (e.g., CCDs), as well as to its applications, and more particularly, to an imaging lens, an image reader, and an imaging apparatus using the image reader.
2. Description of the Related Art
In general, image readers applied to imaging apparatuses, such as digital photocopiers, image scanners, or facsimile machines, optically read the image information on an original, and convert the optical image to electric signals. The optical image reflected from the original is reduced and focused on a solid-state image pickup device, such as a CCD array or a line sensor, using the imaging lens furnished in the image reader.
FIG. 1 schematically illustrates an example of the conventional image reader assembled in a digital photocopier. The original 1006 placed on the stage 1001 is scanned in an optical manner. The original is illuminated by the light source 1002a, and the first carriage 1002 including the light source 1002a and the first mirror 1002b moves in the sub scanning direction (i.e., in the feeding direction). The light reflected from the original is guided via the first mirror 1002b to the second mirror 1003a of the second carriage 1003 that moves in the sub scanning direction at a half speed of the first carriage 1002. The second mirror 1003a reflects the light toward the third mirror 1003b, which then guides the light to the imaging lens 1004. The light image through the imaging lens 1004 comes to focus on the CCD 1005.
The CCD 1005 includes one or more lines of light-receiving elements arranged in the main scanning direction (i.e., orthogonal to the feeding direction), and it converts the light image of the original into electric read signals.
The conventional imaging lens 1004 is designed so as to have a certain resolution over a prescribed threshold level at each image height. However, the actual resolution levels vary among image heights due to variation in the imaging positions (or the focusing positions) on the solid-state image pickup device (e.g., the CCD 1005), especially in the main scanning direction and the sub scanning direction (orthogonal to the main scanning direction). This results in the signal levels output from the light-receiving elements of the solid-state image pickup device (CCD 1005) also differing from each other even if the original has a uniform tone (or density).
The above-described problems in the conventional imaging lens and the image reader are serious because the reproduced image quality deteriorates, and therefore, a technique for outputting more uniform signals from the CCD array at each image height is desired.
Focus adjustment for the optical system of an image reader is generally carried out using a test sheet with black and white stripes. The stripes extend breadthwise (or in the main scanning direction), and are arranged at an equal interval in the sub scanning direction (orthogonal to the main scanning direction). This test sheet is placed on the glass stage 1001 and scanned. The positional relationship between the CCD (or the line sensor) 1005 and the imaging lens 1004 is adjusted so that the magnification (or the reduction ratio) and the output from the CCD (or the line sensor) 1005 are optimized.
However, with this method, only the signal outputs in the main scanning direction of the CCD 1005 are observed during the focus adjustment. If the focus adjustment is carried out based on MTF (modulated transfer function) profiles as illustrated in FIG. 2, inappropriate position A may be selected as a reference to adjust the positional relation between the imaging lens 1004 and the CCD 1005. At position A, which resides in the main (horizontal) OK region, the on-axis resolution indicated by the bold line and the resolution in the main scanning direction indicated by the dashed line exceed the threshold level required to satisfy the spec. However, the resolution in the sub scanning direction indicated by the solid line does not reach the threshold level, and consequently, the resolution of the reproduced image deteriorates in the sub scanning direction. This is because position A shown in FIG. 2 resides also in the sub NG region, and because the MTF profile in the sub scanning direction can not be observed simultaneously with the MTF profile in the main scanning direction during the focus adjustment. If it is found after the focus adjustment that the resolution in the sub scanning direction is insufficient, the positional relation between the imaging lens 1004 and the CCD 1005 has to be adjusted again, and the adjusting process becomes troublesome as a whole.
Japanese patent application laid-open publication No. 8-214112 discloses a focus adjustment technique that can achieve a high resolution. However, with this method, focusing is carried out based on the pre-known performance of an imaging lens, without observing the MTF profile in the sub scanning direction. If the performance of the actually used imaging lens differs from the reference due to variation in the components of the imaging lens within the tolerance, then the focal point can not be correctly brought into the designed position at which the resolutions become optimum in both the main and sub scanning directions.
Another method for achieving high-resolution reproduction is disclosed in Japanese patent application laid-open publication No. 2000-307828. In this publication, the image reader uses an imaging lens including an anamorphic lens. In this method, the imaging lens is rotated about its optical axis in order to correct imbalance between the MTF profiles at left and right image-heights. Such imbalance in the MTF profiles is due to eccentricity of the imaging lens caused during lens assembling, and it turns up as an offset of the focal point due to inclination of the image plane. This method can not deal with variations in the sub scanning direction because the tilt of the image plane in the main scanning direction can not be adjusted in the sub scanning direction.
Still another problem to be considered is color adjustment in a color photocopy. A conventional color photocopier uses a 3-line CCD having red (R), green (G), and blue (B) filters, which separate the original color image into three primary colors (R, G, B). In this case, the focal positions of the respective color (R, G, B) images must agree with each other.
FIG. 3 illustrates an imaging defect in the conventional imaging lens. Imaging lenses generally have chromatic aberration on their axes. In the example shown in FIG. 3, green is the reference wavelength, red has a positive chromatic aberration, and blue has a negative chromatic aberration. In this case, a good MTF profile satisfactory over these three colors can not be obtained regardless of whichever the imaging positions I1, I2 and I3 the image plane is brought to.
FIG. 4 illustrates MTF profiles of the RGB primary colors with the image plane coincident with the imaging position I1. Only the blue image exhibits a good MTF profile. To overcome the imbalance in the color image, the chromatic aberrations of the imaging range have to be appropriately corrected over a wide range of wavelength. However, it is very difficult to completely correct the chromatic aberrations. Some expensive glass material may reduce chromatic aberrations to some extent; however, the fabrication cost of the imaging lens increases.
Japanese patent application laid-open publication No. 6-326833 discloses a technique for reducing displacement of the focal positions of a color image by inserting a multiplex dichroic mirror in the optical path between the imaging lens and the CCD. Because this method requires a new component (i.e., the multiplex dichroic mirror), an additional new holder mechanism must be prepared, and the total cost becomes high. Furthermore, the multiplex dichroic mirror is inserted between the imaging lens and the CCD, the therefore, the surface precision of the multiplex dichroic mirror must be maintained very high.
This method also requires the optical path length between the imaging lens and the CCD to be adjusted with high accuracy. Accordingly, the angle and the thickness of the multiplex dichroic mirror must be strictly set to designed values. The unit price of each component inevitably rises, and adjusting time becomes long. Still worse is the problem that adjusting the optical path length causes the magnifications to vary among the different colors.